Microcontrollers of the type initially mentioned are used on a large scale, for instance in multimedia devices such as music or video players or for control or measuring purposes. In this context, the test interface of the microcontroller serves for fault diagnosis and the function check during the development phase of application and/or system programs which are supposed to be executed on the microcontroller. In particular, interfaces according to the JTAG specification (Joint European Test Action Group) developed by the Institute of Electrical and Electronics Engineers (IEEE) and published under the designation IEEE 1149.1 have become established as test interfaces.
The test interface, however, involves the risk that any data stored within the microcontroller or processed by the microprocessor of the microcontroller might be read out by unauthorized parties even after completion of the development phase. This may be problematic in particular with those microcontrollers which are used in the multimedia domain and are adapted for processing multimedia data (pieces of music, movies etc.), which are subject to the so-called Digital Rights Management (DRM).